The present invention provides a two-stage melt polymerization process for the production of high molecular weight polybenzimidazoles utilizing a free aromatic dicarboxylic acid as monomer.
High molecular weight polybenzimidazoles are polymers of high thermal stability and excellent resistance to oxidative or hydrolytic degradation, which can be formed into shaped articles such as fibers and films having very desirable physical properties. As taught by the published literature, these polymers may be prepared, for example, by melt polymerizing an aromatic tetraamine and a diphenylester or an anhydride of an aromatic or heterocyclic dicarboxylic acid in one or two stage process; see, for example H. Vogel and C. S. Marvel, Journal of Polymer Science, Vol. L, pages 511-539 (1961); and U.S. Pat. Nos. Re. 26,065; 3,174,947; 3,509,108; 3,551,389; 3,433,772; and 3,655,632. In particular, U.S. Pat. No. 3,555,389 discloses a two-stage process for the production of aromatic polybenzimidazoles, in which the monomers are heated at a temperature above 170.degree. C. in a first stage melt polymerization zone until a foamed prepolymer is formed. The foamed prepolymer is cooled, pulverized, and introduced into a second stage polymerization zone where it is heated again to yield a polybenzimidazole polymer product.
It has also been known to prepare polybenzimidazoles from the free dicarboxylic acid rather than the phenyl esters or anhydrides in a melt polymerization process. However, many of the early investigators of these polymers believed that inferior or unsatisfactory polymers resulted from the use of the free acid as the monomer. Thus Vogel and Marvel state in their cited article that in attempting to prepare polybenzimidazoles from aromatic dioic acids and aromatic tetraamines, "The use of the free acids and the corresponding dimethyl esters gave inferior results." (page 513) Moreover, these same two investigators do not mention the free acids as monomers in their U.S. Pat. No. 3,174,947 in which polybenzimidazoles are disclosed and claimed, nor are the free acids mentioned as possible monomers in any of the other cited patents.
In addition to the melt polymerization process described in the foregoing references, a process of preparing polybenzimidazoles has been proposed in which good results are reported to be obtained using a free dicarboxylic acid as monomer. This process involves the solution polymerization of an inorganic acid salt of an aromatic tetraamine and a dicarboxylic acid or a derivative thereof with heat in the presence of polyphosphoric acid; (see U.S. Pat. No. 3,313,783 and Iwakura et al, Journal of Polymer Science: Part A, Vol. 2, pages 2605-2615 (1964). According to this process, the polymer product after completion of the reaction can be separated by pouring the reaction mixture in the form of a polyphosphoric acid solution into a large quantity of water. However, this separation procedure is complicated and, moreover, it is difficult to recover and reuse the polyphosphoric acid. Thus, this process is not generally considered satisfactory for the commercial production of polybenzimidazoles.
Despite the disadvantage of processes utilizing a free dicarboxylic acid as monomer in the preparation of polybenzimidazoles, there are concomitant advantages to such use over the phenyl esters which are the most widely taught dicarboxylic acid derivatives for the preparation of these polymers. Thus, the free acid is generally less expensive than the corresponding phenyl ester and its use does not result in the formation of free phenol as a by-product. The formation of free phenol is undesirable because of toxicity problems. In view of this there have been attempts to develop processes which utilize a free acid as monomer and result in polymers which can be formed into shaped articles of commercial utility. For example, U.S. Pat. No. 4,312,976 discloses a single-stage process of preparing polybenzimidazoles from a free dicarboxylic acid as monomer, wherein the catalyst is an organic sulfonic acid, a halogenated acetic acid or a non-oxidizing inorganic acid. However, aside from being limited to a specific class of catalysts, this process results in a polymer mass of high viscosity requiring substantial agitation energy especially in the latter part of the polymerization process. Furthermore, the final polymer is in the form of a solidified mass which is difficult to handle and to dissolve in a appropriate solvent for forming into shaped articles such as fibers.